1. Field of the Invention
The invention relates to a built-up airgap-insulated exhaust manifold of an exhaust system of a motor vehicle and to a method for producing it.
2. Description of Prior Developments
A built-up airgap-insulated exhaust manifold is known, for example, from DE 195 11 514C1. This reference describes the manufacture of an exhaust manifold which consists of a plurality of inner pipes plugged with a sliding fit one into the other, of an outer casing and of inlet flanges and an outlet flange. The outer casing is produced with a semimonocoque design. First, the plugged-together assembly of the inner pipes (pipe bend, T-piece and branched pipe with connection to the outlet flange) is inserted into a lower outer-casing half-shell. Then, the upper half-shell is pressed onto the lower half-shell and welded to the lower half-shell so as to form a beading seam between the inner-pipe ends.
The plugged-together assembly of the inner pipes is centered within the outer casing in a complicated way by means of special spacer rings which are pushed onto a plurality of inner pipes, the gap which at the same time occurs forming the subsequent air insulation gap. The spacer rings consist of a material which decomposes and/or sublimates under the action of heat, in particular when the engine is in operation. The individual pipes are subject to manufacturing tolerances and are displaceable relative to one another. Because of the work involved in fixing the plugged-together assemblies to one another, the assemblies have different xe2x80x9cpluggingxe2x80x9d lengths. The spacer rings are themselves subject to manufacturing tolerances and as a result of their shape in relation to the design of the lower shell, seldom come to bear continuously against the latter. For these reasons alone the manufacture of the entire exhaust manifold is affected by tolerances.
In view of the manufacturing tolerances mentioned, the inner pipe having the branch connection piece virtually never lies within the outer casing with the desired defined continuous airgap. Accurate reproducibility does not exist in this case.
At the same time, it is necessary, during assembly, to ensure that a specific minimum plugging length is adhered to, so that the individual inner pipes do not slip out of one another. This adherence necessitates judgment by the eye and therefore considerable effort. During the transfer of parts to the welding station, vibrations and centrifugal forces may also occur, which cause the individual inner pipes once again to be displaced relative to one another and to the lower shell of the outer casing, and this may even lead to the plugged-together assembly coming loose.
Due to the rebound distortion of the two sheet-metal half-shells after deep-drawing, the two outer-casing half-shells do not by themselves bear against one another in a continuously snug and gap-free manner. In the welding station, therefore, the upper shell of the outer casing is placed onto the lower shell and is pressed against this. In this case, too, there are vibrations of the plugged-together assembly or displacement at the relative position of the branched inner pipe in the outer casing.
Finally, the shells of the outer casing are laser-welded to one another. After the applied pressure has been cancelled, considerable tensile forces then act on the weld seam because of the non-uniformity of the bearing faces of the half-shells. This lowers the permanent load-bearing capacity of the assembly as a whole, in particular of the outer casing, and may even lead to a failure of the structural part when the exhaust tract is in operation. Thus, overall, process reliability in the production of the exhaust manifold is not ensured to a sufficient extent.
The welding together of the half-shells so as to form a beading seam is also relatively complicated, particularly since a triangular gusset occurs, because of edge radii, at the transition to the cutout of the outer casing for the branched connection piece of the inner pipe. This gusset has to be welded shut for process reliability, and in practice, can be done in an appropriate way only with the aid of an additional material. Moreover, the beading seam, because of its configuration, has a limited mechanical load-bearing capacity. For fixing the inner pipe to outer casing, additional welding is necessary, so as to form a round seam. That is, to form a continuous fillet seam in the end region of the branch connection piece. The end of the inner pipe of the connection piece is set back somewhat relative to the orifice of the outer casing.
Because of the branched exhaust pipe, the outer casing is designed with a very long spatial projecting length, since the production of the half-shells by deep-drawing means that a branch cannot be achieved and it is therefore unsuitable for designing an outer casing with a contour true to the configuration of the inner pipe. All the inner pipes are, in this case, surrounded integrally by a single common outer casing, thus resulting, because of the uniform termination of the outer casing approximately in the plane of the inlet flanges, in relatively large-volume sheet-metal portions of the outer casing. These large volume portions occur between the inner pipes adjoining the inlet flanges and require considerable construction space, increase the weight of the branched exhaust pipe and entail additional unnecessary outlay in terms of material. Moreover, it is consequently not possible to produce a defined evenly uniform airgap at the branched exhaust pipe.
Furthermore, engines having different numbers of cylinders require exhaust manifolds which are designed differently on account of the outer casing. This signifies a high additional outlay in terms of manufacture and tooling, along with the corresponding costs. Likewise, for differently designed construction spaces, new variants in the semimonocoque design of the exhaust manifold, which are adapted to these construction spaces, have to be invented. Implementing this likewise necessitates a considerable outlay in manufacturing terms.
The object on which the invention is based is to provide an airgap-insulated exhaust manifold which saves construction space and weight, and to provide a production method for this, by means of which a process-reliable and exactly reproducible design of the exhaust manifold can be achieved in a simple way.
The invention makes it possible to have a modular design of the exhaust manifold, in which exhaust manifolds configured in any desired way in terms of their extent and overall depth can be manufactured in the simplest possible way from airgap-insulated individual exhaust pipes plugged one into the other, with the outer pipes of the individual exhaust pipes being welded to one another and the inner pipes being positioned with a sliding fit one in the other. The individual modules form the individual exhaust pipes which constitute standard structural elements and therefore mass articles capable of being produced cost-effectively. Thus, simply by joining together identical parts of the branched exhaust pipes, it is possible, for example from a four-cylinder exhaust manifold, to produce a 6-cylinder or 8-cylinder exhaust manifold.
The entire assembly operation is simplified substantially by the use of identical parts. Since the individual exhaust pipes are manufactured by means of internal high-pressure forming, this avoids any manufacturing tolerances which result from inner pipes located in the plugged-together assembly being displaced during the individual assembling and joining steps, so that any desired exhaust manifold can be reproduced exactly. The lack of an integral outer casing and the fastening of outer-casing half-shells to one another and of the outer casing to the inlet flanges mean that the difficulties caused by mechanical and thermal stresses in the weld seams hitherto necessary are avoided.
Since the outer pipe is designed with a contour true to the run of the inner pipe or its shape by means of a double pipe formed by internal high pressure into an airgap-insulated exhaust pipe, superfluous material of the outer pipe is avoided, in contrast to the outer casing of the semimonocoque design, and the construction space is thereby also reduced. Overall, the design of the exhaust manifold can be adapted flexibly to the shape of the construction space provided, since the individual exhaust pipes of the manifold can follow the run of the construction space by being suitably lined up with one another.
By contrast, the exhaust manifold in the semimonocoque design would be so bulky, because the pipes carrying exhaust gas run into the depth of the construction space, that installation is impossible from the outset. Furthermore, by the exhaust pipes being manufactured by means of internal high-pressure forming, the air insulation gap can be set in a controlled manner, and everywhere uniformly, over the entire extent of the exhaust pipe.
The points where the outer pipes are joined to one another are welded together, preferably by means of a laser, so as to form a continuous fillet weld seam having very high mechanical load-bearing capacity. Overall, high process reliability is achieved by means of the production method according to the invention, since, by virtue of the internal high-pressure forming, there is no possibility of displacing the inner pipes so as to cause the plugged-together assemblies to become loose and, the number of weld seams is minimized. The exhaust manifold is configured in such a way that only continuous fillet seams, which are simple to execute and are capable of being subjected to mechanical stress, are necessary for fastening the individual exhaust pipes to one another and to the inlet flanges and the outlet flange.
In the airgap insulation of the exhaust pipes, there is no need for the pipe bend to be formed by internal high pressure, this being advantageous in terms of tooling. Furthermore, the partial airgap insulation of the exhaust manifold affords advantages due to the essentially simpler production of the individual exhaust pipes which are free of an air insulation gap. The reduction in complexity of the design of the exhaust pipes brings about a further improvement in process reliability. The exhaust pipes free of an air insulation gap can, of course, be used only where there are no heat-sensitive parts of the motor vehicle arranged in the immediate vicinity. Moreover, because the second pipe is dispensed with in a single-walled exhaust pipe free of an air insulation gap, the exhaust manifold is substantially lighter and also requires less construction space.